A known disadvantageous property of turbocharged Diesel engines is that the characteristics of the turbocompressor output are unfavorable to the air requirements of a piston engine both with respect to the engine itself and with respect to a machine unit driven by the exhaust turbine. The disadvantage is that at low engine r.p.m. the turbocharger air delivery is less than that required while in a higher r.p.m. range there is a risk of overcharging, and thus excessive heat stress (thermal load) of the engine owing to the air delivery being greater than that required. Because of the changing r.p.m. conditions in the operation of motor vehicles, these disadvantageous characteristics arise to a greater extent with them and there are attendant smoke and pollutant emission characteristics, especially on acceleration. Several known proposals have aimed at eliminating the above unfavorable properties of turbocharged engines.
In one known construction, upstream of the exhaust gas turbine, there is provided in the exhaust pipe a by-pass duct by-passing the gas turbine and closable by a valve which is controlled by a pressure sensor coupled to the pressure duct of the turbocompressor and which passes a portion of the exhaust gas into the by-pass duct whenever the charging pressure of the turbocompressor exceeds a predetermined value. But a difficulty with this apparatus is that a regulating valve is installed in the exhaust pipe which is subjected to high temperature and that the turbocharger cannot be operated at higher engine r.p.m. in the high-efficiency range of its characteristic and also that a part of the energy of the exhaust gas remains unutilized.
In another known construction the delivery duct of the turbocompressor is closable by valves and is connected with the section of the exhaust duct upstream of the turbine and thus part of the compressed air can be transferred to mix with the exhaust gases, e.g. for feeding one of the two turbines, setting of the valve is effected by a hand lever or automatically in dependence on engine r.p.m. This method can only be employed only for large engines partly because of the difficulty of handling and partly because of the use of two turbines.
Another construction that can be employed for large and relatively slowly operating engines involves two compressors on a common shaft. At low engine r.p.m. operation only one of the compressors is operative while the suction and delivery ducts of the other compressor are closed by a respective butterfly valve.
In yet another construction it is sought to improve the inadequate air delivery at low engine r.p.m. by connecting the delivery duct of the turbocompressor to terminate in a collecting or plenum tank connected to a resonance pipe with a resonance tank. The latter is in turn coupled to the short suction pipes of the engine cylinders, the suction periods of which do not overlap each other to any significant extent; the natural resonance frequency of the system is tuned to an engine r.p.m. which is so low as to assist in increasing the otherwise insufficient air output of the turbocharger. A limitation to the use of this method is defined by the size of the effective resonance range; also the dimensions of the apparatus are considerable and can frustrate its use, in certain cases, for vehicle engines.